"How can you tell if you got one from a bad batch? It doesn't sharpens up when you use smaller apertures at some focal lengths or what?"
I noticed on some shots I had taken in Paris that there was pronounced lateral softness. It was only a few images out of a couple of hundred, and I was not sure of the focal length or f-stop on those shots. So I tested it systematically, 28, 35, 50, and 80 mm., all f stops to 16. It turns out that the weakness was at 50 mm. f5.6, and a bit at f8.0. When I got the replacement I also tested it systematically and found it was superb.
The test was on a tripod, using Provia 100F. I then scanned the images on a Nikon LS 2000 and looked at them at 200% to 300% magnification in Photoshop, no sharpening.
Then, of all things, the Tokina techs said nothing about it being soft, but said that it had low contrast. I had not complained about that. A batch went out with a bad rear element, or so I am told. Tokina tried first to replace the rear element group, then gave that up and sent me a replacement.
So if you buy a used one, test it systematically.
Joe
